The objective of this research program is to provide information on the functioning and maintenance of the vertebrate photoreceptor. This program emphasizes the mechanism of action of the rod visual pigment, rhodopsin. In providing basic information on the functioning of the vertebrate photoreceptor, this program may also indicate the basis of operation of other receptor, neurobiological, or hormone systems. A variety of chemical, biochemical, and electrophysiological techniques are used. Microspectrophotometry, regular spectrophotometry, disc gel electrophoresis, intracellular or extracellular electrical recording and measurements of cell respiration are some of the methods used. The samples used include excised vertebrate retinas, solution extracts of visual pigment and excised eyes from visually deficient Drosophila (fruitflies). In the vertebrate, data have been obtained on the conformational changes of rhodopsin after illumination. These include changes in the pK's of certain ionizable groups, location and light-induced exposure of sulfhydryl groups, and the sequence and kinetics of a number of reactions. In visually deficient fruitflies, data have been obtained on the protein components of the eye and the chemical and kinetic properties of the rhodopsin. In both systems the relationships between these properties and the electrical and chemical events of the photoreceptor have been investigated.